Simulated Tropical Cyclone Intensity Research Articles

Influence of surface layer schemes on tropical cyclone Hato (2017) intensity

Three surface layer schemes (MYNN, ETA, and MM5) in the Weather Research and Forecasting (WRF 4.1) model were selected to conduct simulation experiments for tropical cyclone (TC) Hato (2017). The influence of different surface layer schemes on TC intensity was compared. Although the planetary boundary layer and land surface schemes were the same, surface layer schemes minimally affected TC tracks but significantly affected TC intensity. The highest correlation coefficient and smallest bias and root mean square error between the simulated TC intensity in the ETA and observations. The TC intensity simulated using ETA was the strongest and closest to the observations. The surface latent heat fluxes simulated using ETA at the time of the TC landing were 1200–1500 W/m2, and is 800–1200 W/m2 by MM5 and MYNN, respectively. The surface sensible heat flux simulated using ETA was 400–800 W/m2 and that simulated using MM5 and MYNN was 200–400 W/m2. The equivalent potential temperature and radial wind simulated using ETA were significantly higher than those simulated using MM5 and MYNN. The larger the energy transmitted from the surface to the tropical cyclone, the stronger the TC. The dynamic structural characteristics were consistent with the tropical cyclone intensity. The ETA indicated the strongest unstable planetary boundary layer structure, stronger convergence at the lower layer, divergence at the upper layer, vertical transport, and absolute angular momentum, and induced the formation of the strongest TC. The comparison results revealed that the surface fluxes from different surface layer schemes can affect the dynamic and thermal structures of the planetary boundary layer owing to the different stability functions. The surface fluxes were also affected by the temperature and wind in the planetary boundary layer. This interaction between the surface layer and planetary boundary layer affects the eye structure and secondary circulation of the TC and TC intensity.

Effects of Surface Wave-Induced Mixing and Wave-Affected Exchange Coefficients on Tropical Cyclones

Surface waves perform a crucial role in modulating tropical cyclone (TC) systems and have proved to be key for numerical TC predictions. In this study, we investigate the effects of wave-induced mixing and wave-affected surface exchange coefficients using a coupled ocean–atmosphere–wave model for two real TC cases: Shanshan (2018) and Megi (2010). The results demonstrate that wave-affected surface exchange coefficients enhance air–sea heat fluxes and have a significantly positive effect on simulated TC intensity, size, and strengthening process. In contrast, the wave-induced mixing has a negative impact on TC intensity and size and is not conducive to TC intensification and maintenance. The net effect of these two factors is a balance between the wave-affected positive contribution and the negative contribution from wave-induced sea surface temperature cooling. We find that the effect of wave-induced mixing on a TC system depends on the local thermal structure of the ocean. When the thermocline is weak and there is warm water at the wave-induced mixing penetration depth, the negative effect of the wave-induced mixing is weak. However, when there is cold subsurface water and a strong thermocline, wave-induced mixing has a significant impact, which exceeds the wave-driven positive feedback.

The Consequences of Surface-Exchange Coefficient Uncertainty on an Otherwise Highly Predictable Major Hurricane

Abstract In addition to initial conditions, uncertainty in model physics can also influence the practical predictability of tropical cyclones. In this study, the influence that various magnitudes of uncertainty in the surface exchange coefficients of momentum (Cd) and enthalpy (Ck) can have on an otherwise highly predictable major hurricane (Hurricane Patricia) is compared with that resulting from climatological environmental initial condition uncertainty and the intrinsic limit for this case. As the systematic uncertainty in Cd and Ck is reduced from 40% to 1%, the simulated uncertainty in the intensity and structure is substantially reduced and approaches the intrinsic limit when uncertainty is reduced to 1%. In addition, the forecasted intensity and structure uncertainty only becomes less than that resulting from climatological environmental initial condition uncertainty once the systematic uncertainty in Cd and Ck is reduced to ∼10%, highlighting the strong influence of model error in limiting TC predictability. If Cd and Ck are perturbed stochastically, instead of systematically, it is shown that the influence on the simulated intensity and structure is negligible and nearly identical to the intrinsic limit, regardless of the magnitude of the stochastic Cd and Ck perturbations. While the magnitude of the stochastic Cd and Ck perturbations are comparable to the systematic perturbations, the stochastic perturbations are shown to not substantially perturb the time-integrated inner-core fluxes of momentum or enthalpy that predominantly determine simulated tropical cyclone intensity. Last, it is shown that the kinetic energy error growth behavior varies with the radius, azimuthal wavenumber, and ensemble design. Significance Statement The air–sea energy exchange beneath hurricanes is highly uncertain but strongly influences intensity. In this study, the influences of different magnitudes of surface-exchange coefficient uncertainty on the simulated intensity of an intense hurricane is compared with that resulting from environmental initial condition uncertainty and the intrinsic predictability limit. The main takeaway is that current surface-exchange coefficient uncertainties result in larger intensity uncertainty than environmental initial condition uncertainty, and substantial improvements in predictions are possible if current surface-exchange coefficient uncertainties are reduced. Furthermore, it is shown that randomly perturbing the surface-exchange coefficients at each point in space and time is not the best approach to account for the influences of this uncertain physical process on hurricane prediction because it has minimal influence on the simulated intensity.

A New Time-Dependent Theory of Tropical Cyclone Intensification

AbstractIn this study, the boundary layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer, is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal-wind balance, with eyewall convection coming into moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies, as found recently based on idealized numerical simulations. An ad hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic Ocean and eastern and western North Pacific Ocean. As compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as seen in observations.

Maximizing Simulated Tropical Cyclone Intensity With Action Minimization

AbstractDirect computer simulation of intense tropical cyclones (TCs) in weather models is limited by computational expense. Intense TCs are rare and have small‐scale structures, making it difficult to produce large ensembles of storms at high resolution. Further, models often fail to capture the process of rapid intensification, which is a distinguishing feature of many intense TCs. Understanding rapid intensification is especially important in the context of global warming, which may increase the frequency of intense TCs. To better leverage computational resources for the study of rapid intensification, we introduce an action minimization algorithm applied to the Weather Research and Forecasting and WRFPLUS models. Action minimization nudges the model into forming more intense TCs than it otherwise would; it does so via the maximum likelihood path in a stochastic formulation of the model, thereby allowing targeted study of intensification mechanisms. We apply action minimization to simulations of Hurricanes Danny (2015) and Fred (2009) at 6‐km resolution to demonstrate that the algorithm consistently intensifies TCs via physically plausible pathways. We show an approximately tenfold computational savings using action minimization to study the tail of the TC intensification distribution. Further, for Hurricanes Danny and Fred, action minimization produces perturbations that preferentially reduce low‐level shear as compared to upper‐level shear, at least above a threshold of approximately 4 m/s. We also demonstrate that asymmetric, time‐dependent patterns of heating can cause significant TC intensification beyond symmetric, azimuthally averaged heating and find a regime of nonlinear response to asymmetric heating that has not been extensively studied in previous work.

Halving warming with idealized solar geoengineering moderates key climate hazards

Solar geoengineering (SG) has the potential to restore average surface temperatures by increasing planetary albedo1–4, but this could reduce precipitation5–7. Thus, although SG might reduce globally aggregated risks, it may increase climate risks for some regions8–10. Here, using the high-resolution forecast-oriented low ocean resolution (HiFLOR) model—which resolves tropical cyclones and has an improved representation of present-day precipitation extremes11,12—alongside 12 models from the Geoengineering Model Intercomparison Project (GeoMIP), we analyse the fraction of locations that see their local climate change exacerbated or moderated by SG. Rather than restoring temperatures, we assume that SG is applied to halve the warming produced by doubling CO2 (half-SG). In HiFLOR, half-SG offsets most of the CO2-induced increase of simulated tropical cyclone intensity. Moreover, neither temperature, water availability, extreme temperature nor extreme precipitation are exacerbated under half-SG when averaged over any Intergovernmental Panel on Climate Change (IPCC) Special Report on Extremes (SREX) region. Indeed, for both extreme precipitation and water availability, less than 0.4% of the ice-free land surface sees exacerbation. Thus, while concerns about the inequality of solar geoengineering impacts are appropriate, the quantitative extent of inequality may be overstated13. High-resolution coupled climate model simulations suggest only 0.4% of the land surface will see exacerbated hydrological risks under solar geoengineering that halves warming, indicating that geoengineering-related risks may be overstated.

Impacts of Radiation and Upper-Tropospheric Temperatures on Tropical Cyclone Structure and Intensity

Abstract Potential intensity theory predicts that the upper-tropospheric temperature acts as an important constraint on tropical cyclone (TC) intensity. The physical mechanisms through which the upper troposphere impacts TC intensity and structure have not been fully explored, however, due in part to limited observations and the complex interactions between clouds, radiation, and TC dynamics. In this study, idealized Weather Research and Forecasting Model ensembles initialized with a combination of three different tropopause temperatures and with no radiation, longwave radiation only, and full diurnal radiation are used to examine the physical mechanisms in the TC–upper-tropospheric temperature relationship on weather time scales. Simulated TC intensity and structure are strongly sensitive to colder tropopause temperatures using only longwave radiation, but are less sensitive using full radiation and no radiation. Colder tropopause temperatures result in deeper convection and increased ice mass aloft in all cases, but are more intense only when radiation was included. Deeper convection leads to increased local longwave cooling rates but reduced top-of-the-atmosphere outgoing longwave radiation, such that the total radiative heat sink is reduced from a Carnot engine perspective in stronger storms. We hypothesize that a balanced response in the secondary circulation described by the Eliassen equation arises from upper-troposphere radiative cooling anomalies that lead to stronger tangential winds. The results of this study further suggest that radiation and cloud–radiative feedbacks have important impacts on weather time scales.

Comparison of Simulated Tropical Cyclone Intensity and Structures Using the WRF with Hydrostatic and Nonhydrostatic Dynamical Cores

This study explored the influence of choosing a nonhydrostatic dynamical core or a hydrostatic dynamical core in the weather research and forecasting (WRF) model on the intensity and structure of simulated tropical cyclones (TCs). A comparison of cloud-resolving simulations using each core revealed significant differences in the TC simulations. In comparison with the nonhydrostatic simulation, the hydrostatic simulation produced a stronger and larger TC, associated with stronger convective activity. A budget analysis of the vertical momentum equation was conducted to investigate the underlying mechanisms. Although the hydrostatic dynamical core was used, the vertical motion was not in strict hydrostatic balance because of the existence of the vertical perturbation pressure gradient force, local buoyancy force, water loading, and sum of the Coriolis and diffusion effects. The contribution of the enhanced vertical perturbation pressure gradient force was found to be more important for stronger upward acceleration in the eyewall in the hydrostatic simulation than in the nonhydrostatic simulation. This is because it leads to intensified convection in the eyewall that releases more latent heat, which induces a larger low-level radial pressure gradient and inflow motion, and eventually leads to a stronger storm.

A modeling study on the effects of MJO and equatorial Rossby waves on tropical cyclone genesis over the western North Pacific in June 2004

This study investigates the influence of the Madden Julian oscillations (MJO) and equatorial Rossby (ER) waves on tropical cyclone (TC) formation in western Pacific during June 2004 through one control and three wave experiments for each of the five TCs. The control experiment reasonably simulates the formation of five TCs. In the corresponding wave experiments, the MJO, ER waves, and both the MJO and ER waves are removed, from the initial fields and lateral boundary conditions, respectively. The differences of simulated TC intensity between the control and corresponding wave experiments provide a quantitative assessment of the relative contribution of each wave to TC formation.In the wave experiments with the MJO removed, three of the five TCs are weakened, and the remaining two (TC A and B) grow stronger due to an altered background flow that steered the TCs into more favorable oceanic regions. For the wave experiments with ER waves removed, three of the five simulated TCs become weaker (TC A, C, and E). TC D develops into a tropical storm because of a dominant influence from active synoptic-scale disturbance. The results indicate that both the MJO and ER waves have an important modulating effect on TC formation. In addition to the influence from the MJO, ER and synoptic-scale waves, local processes may dominate in TC formation; for the example of TC B, none of the waves positively influence the formation in significant ways. The present modeling approach provides a quantitative assessment of the relative contribution of tropical wave disturbances to TC formation.

Performance of a New Convective Parameterization Scheme on Model Convergence in Simulations of a Tropical Cyclone at Grey-Zone Resolutions

Abstract The latest version of the Weather Research and Forecasting model (WRFV3.5) is used to evaluate the performance of the Grell and Freitas (GF13) cumulus parameterization scheme on the model convergence in simulations of a tropical cyclone (TC) at gray-zone resolutions. The simulated TC intensity converges to a finite limit as the grid spacing varies from 7.5 to 1 km. The reasons for the model convergence are investigated from perspectives of subgrid-scale processes and thermodynamic and dynamic structures. It is found that the impacts of above factors are notably different with varying model resolutions. The convective heating and drying increase as the grid spacing decreases, which inhibits the explicit microphysical parameterization preventing the simulated TC from overly intensifying. As the grid spacing decreases from 7.5 to 5 km, the TC intensity increases because of a stronger secondary circulation, a larger magnitude and proportion of strong eyewall updraft, and a greater amount of latent heating in the eyewall. As the grid spacing decreases from 5 to 3 km, the radius of maximum wind (RMW) decreases and the radial pressure gradient increases leading to an increase in TC intensity. The simulated TC intensity changes slightly as the grid spacing decreases from 3 to 1 km since the RMW and the storm structure both change little. The slight changes in the simulated TC intensity at such high resolutions indicate a great model convergence. Therefore, the GF13 presents an appropriate option that increases the model convergence in the TC intensity simulation at gray-zone resolution.

Sensitivity of the Simulated Tropical Cyclone Inner-Core Size to the Initial Vortex Size*

Abstract The multiply nested, fully compressible, nonhydrostatic tropical cyclone model version 4 (TCM4) is used to examine and understand the sensitivity of the simulated tropical cyclone (TC) inner-core size to its initial vortex size. The results show that although the simulated TC intensity at the mature stage is weakly dependent on the initial vortex size for the general settings, the simulated TC inner-core size is largely determined by the initial vortex size. The initial vortex size is critical to both the energy input from the ocean and the effectiveness of the inward angular momentum transport by the transverse circulation driven by eyewall convection and diabatic heating in spiral rainbands. Strong outer winds in a storm with a large initial size lead to large entropy fluxes to a large radial extent outside the eyewall, favoring the development of active spiral rainbands. Latent heat released in spiral rainbands plays a key role in increasing the low-level radial inflow and accelerating tangential winds outside the eyewall, leading to outward expansion of tangential wind fields and thus increasing the inner-core size of the simulated storm. On the contrary, a storm with a small initial size has weaker outer winds and smaller surface entropy fluxes outside the eyewall and is accompanied by less active spiral rainbands and thus a much slower increase in the inner-core size. The effectiveness of the inward transport of absolute angular momentum to increase the tangential winds outside the eyewall is largely determined by the radial extent of the vertical absolute vorticity, which is shown to be higher in a large size vortex. The relative importance of the initial vortex size and the environmental relative humidity (RH) to the TC inner-core size is also evaluated. It is found that the inner-core size of the simulated storm at the mature stage depends more heavily on the initial vortex size than on the initial RH of the environment.

The Maximum Intensity of Tropical Cyclones in Axisymmetric Numerical Model Simulations

Abstract An axisymmetric numerical model is used to evaluate the maximum possible intensity of tropical cyclones. As compared with traditionally formulated nonhydrostatic models, this new model has improved mass and energy conservation in saturated conditions. In comparison with the axisymmetric model developed by Rotunno and Emanuel, the new model produces weaker cyclones (by ∼10%, in terms of maximum azimuthal velocity); the difference is attributable to several approximations in the Rotunno–Emanuel model. Then, using a single specification for initial conditions (with a sea surface temperature of 26°C), the authors conduct model sensitivity tests to determine the sensitivity of maximum azimuthal velocity (υmax) to uncertain aspects of the modeling system. For fixed mixing lengths in the turbulence parameterization, a converged value of υmax is achieved for radial grid spacing of order 1 km and vertical grid spacing of order 250 m. The fall velocity of condensate (Vt) changes υmax by up to 60%, and the largest υmax occurs for pseudoadiabatic thermodynamics (i.e., for Vt > 10 m s−1). The sensitivity of υmax to the ratio of surface exchange coefficients for entropy and momentum (CE/CD) matches the theoretical result, υmax ∼ (CE/CD)1/2, for nearly inviscid flow, but simulations with increasing turbulence intensity show less dependence on CE/CD; this result suggests that the effect of CE/CD is less important than has been argued previously. The authors find that υmax is most sensitive to the intensity of turbulence in the radial direction. However, some settings, such as inviscid flow, yield clearly unnatural structures; for example, υmax exceeds 110 m s−1, despite a maximum observed intensity of ∼70 m s−1 for this environment. The authors show that turbulence in the radial direction limits maximum axisymmetric intensity by weakening the radial gradients of angular momentum (which prevents environmental air from being drawn to small radius) and of entropy (which is consistent with weaker intensity by consideration of thermal wind balance). It is also argued that future studies should consider parameterized turbulence as an important factor in simulated tropical cyclone intensity.